Black sacrifices a two pieces for Rook and Pawn.
Nominally this is equal but this normally favours the pieces - here
Black may have been encouraged by the doubled Rooks, but they bite
on granite on d4 while the White Bishops are too powerful

17... Bxc4 18. Rxc4 Nxc4 19. Qxc4

19... b6?

Hugh Alexander had a pet generalisation that players only
make blunders in losing positions. I don't know if Black is
actually lost but nothing seems attractive, while White has an
obvious plan of parking the Knight on c5.

[19... e5 is the only active plan for Black, but it
doesn't look promising here

20. Nc5 [20. Bh3]

20. Bxc6 Rc8 21. Rc1 Bf8 22. Qe2 Rdc7 23. d5 e6
24. Bxf8 Kxf8 25. e4

White maintains his bind with some simple moves, which
must have been very disheartening for Black!

But this is unusual - Black may take the knight without
prompting. 5.a3 is most dangerous when White has the plan f3 and e4
available. I think this is a 'typical' difference, in that I would
expect a Major player to have a concrete plan in mind